Many industrial processes require uniform heating of work pieces. Often, the work pieces must be heated to an elevated temperature in order to accelerate a reaction. Heating a work piece in a vacuum presents particularly difficult problems because the heat transfer efficiency is reduced as the vacuum increases. Many of the processes which are used in the semiconductor industry require tremendously pure environments to avoid contaminating the work piece, e.g., semiconductor substrate, from deleterious side reactions, by products, or heating apparatus degradation contaminants.
Previous apparatus for heating substrates in a vacuum environment encase a standard heater wire in a silica quartz tube. The heater is placed above the substrate and energized to heat the substrate. Because the heating element and the silica or quartz tube expand at different rates because of their different thermal coefficients of expansion, the seal between the silica or quartz shield and the heating element develops micro cracks. When the seal between the heating element and silica or quartz tube breaks this permits the pressure within the tube to rise to ambient conditions when the vacuum environment is terminated. The lack of pressure within the tube equalizes with the surrounding environment by the uptake of gases in the chamber. When the apparatus is used again, the out gassing of the contaminants from the tube results in memory problems and other deleterious side effects to the environment within the vacuum chamber. This destroys or degrades the purity of the vacuum and can cause contamination of the semiconductor device being fabricated.
III-V compound semiconductor such as GaAs/A1GaAs devices are of particular interest because of their increased efficiency. Standard quartz heater tubes present particular problems with the fabrication of these devices because the gases used to create the layers in the device react with the silica or quartz. This degrades the environment and increases the problems associated with fabricating high quality semiconductor devices. In addition, the standard copper heater contacts on the outside of the heating lamps forms a gallium copper alloy which quickly reduces efficiency of the heater and drastically shortens its lifetime.
Thus, it would be highly desirable to have an apparatus which can uniformly heat the semiconductor substrate without exposing the environment to unnecessary contaminants or residual dopants from previous manufacturing processes. It would also be desirable to have an apparatus having a modular construction so that individual components can be easily and quickly replaced to minimize the down time of the apparatus.